1. Field of the Invention
The present invention relates to a shutter device for a camera. More particularly, the present invention relates to a shutter device for a camera in which shutter blades can open without failure even when exposure time is very short.
2. Description Related to the Prior Art
Some types of compact cameras or instant cameras incorporate a program shutter, of which shutter blades are used also for controlling an exposure amount. A program shutter is constructed to take an exposure according to a predetermined combination of an aperture stop and a shutter speed respectively for each exposure value. Two or more shutter blades are combined and simultaneously opened/closed by means of a motor, a solenoid or other actuators. Open time or timer time of the shutter blades is determined in accordance with the exposure value. An opening diameter of the shutter blades corresponds to the aperture stop. A moved amount of the shutter blades is determined by the timer time. The opening diameter is increased and decreased according to the moved amount of the shutter blades. This is a widely used technique with considerable simplicity and convenience.
In FIG. 15, states of driving the program shutter according to the prior art are illustrated. A curve S4 indicates a relationship between an elapsed time and a change in a moved position (the moved amount) of the shutter blades. Sign D0 designates a moving position where a shutter opening starts being formed between the shutter blades. When exposure value EV is calculated by photometric operation, then the timer time T0 is uniquely determined. When a trigger signal is generated by a user's releasing operation of the shutter device, the motor starts rotating in a forward direction. Then the shutter blades are moved in an opening direction. Upon the start of the forward rotation of the motor, the timer time T0 starts being measured.
When the moved amount of the shutter blades increases for the shutter blades to come to the moving position D0, then the shutter blades starts forming the shutter opening. When the moved amount further increases, the opening diameter also increases. Upon the finish of measuring the timer time T0, a shutter-close signal is generated. The motor, in response to the shutter-close signal, is caused to rotate in reverse. The shutter blades, having been moved in the opening direction, now move in a closing direction, and finally come back to a closed position. One sequence of opening/closing operation is finished. Note that, immediately after the occurrence of the shutter-close signal, inertia and various factors move the shutter blades farther in the opening direction. Of course one exposure is being taken while the shutter blades are coming in the closing direction. In consideration of those various steps, the timer time T0 is predetermined. It is possible to control the exposure amount only by determining the timer time T0 according to the exposure value EV even with a comparatively simple construction.
However there is a problem in remarkable changes in an operating state of the shutter blades in the course of the opening/closing operation. Those changes are due to various causes including a difference in the orientation of manually holding the instant camera, changes in friction between the shutter blades, changes in friction between a shutter base plate and the shutter blades, differences between plural individual shutter blades, changes of the shutter blades with time, changes in environmental conditions for use of the shutter blades, and changes in response of the actuator. Especially when the shutter blades start being moved in the opening direction, the time point of the start of the shutter blades or the moving speed of the shutter blades are likely to change, because the state of rest of the shutter blades changes to the moving state.
If the shutter blades start being moved less quickly or are moved less quickly upon the start of their operation, the shutter blades operate in the manner indicated by a curve S5 of FIG. 15. If an exposure is taken with the timer time T0 associated with the photometrically obtained exposure value EV with the shutter blades of the curve S5, then it results in the under-exposure relative to the exposure with the shutter blades according to the curve S4. Also another most unacceptable state may occur in which the shutter blades come back in the closing direction without forming the shutter opening as indicated in a curve S6. This is remarkably frequent when a subject brightness of a photographic subject is very high. The opening diameter is controlled to be small for high subject brightness, to shorten the timer time T0 to reduce the moved amount of the shutter blades. A problem lies in that no exposure is taken even by exposure-taking sequence, due to failure in the opening operation of the shutter blades.
It may be conceivable that, in order to solve such a problem, a lower limit is predetermined for inhibiting the timer time T0 from coming lower than it. At least the smallest size of the shutter opening could be formed. But this is unacceptable because of ineffectiveness in controlling an exposure with high brightness, which has been possible according to the known techniques.